Abnormal vascular tone and reactivity following the induction of ischemic acute renal failure (ARF) has the potential to modify substantially the course of disease. There are direct data from experimental models and indirect evidence from clinical investigation that hypersensitivity of post-ischemic renal resistance vessels to a variety of vasomotor stimuli can predispose the kidneys to recurrent ischemic injury in the established phase of ARF. In the norepinephrine-induced model of ischemic ARF (NE-ARF), the period of most pronounced hypersensitivity to vasomotor stimuli is at 48 hr after ischemia induction, as demonstrated by markedly increased vasoconstrictor responses to agonists such as angiotensin II (AII) and endothelin-1 (ET-1) and paradoxical constriction to reduction in arterial pressure in the autoregulatory range both in vivo and in isolated renal arterioles. Neither the mechanism of increased basal vascular tone nor the mechanism of altered post-ischemic vascular sensitivity is understood. The overall proposed hypothesis for post-ischemic increased basal vascular tone and abnormal vascular sensitivity involves an increased smooth muscle cell (SMC) membrane Ca2+ leak coupled with increased sarcoplasmic reticulum (SR) storage. In response to SR Ca2+ mobilizing agonists or to reduction in vessel wall tension, there is a super- physiologic SR Ca2+ release and consequent associated exaggerated constriction. To test this hypothesis, isolated afferent (AA) and efferent arterioles (EA) from 48-hr NE-, renal artery clamp (RAC)-, and sham-ARF kidneys will be examined for abnormal SMC [Ca2+] influx and extrusion, increased SR Ca2+ storage and release, changes in SMC membrane potential and K+ channel activity, and alterations in endothelial paracrine vasodilator and vasoconstrictor balance. The goal of these experiments is to determine a pathophysiologic basis for aberrant vasoactivity in post-ischemic ARF that can form a basis for in vivo therapeutic intervention.